The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 2001-256227 filed Aug. 27, 2001.
1. Field of the Invention
The present invention relates to an electronic flash device that emits flash light in synchronization with a camera photographing operation.
2. Description of the Related Art
There are electronic flash devices known in the related art that implement light emission control by connecting a drive element such as an IGBT (insulated gate bipolar transistor) in series with a light emitting tube (see Japanese Laid-Open Patent Publication No. S64-17033 and Japanese Laid-Open Patent Publication No. H4-27164, for instance). This type of electronic flash device implements on/off control of the drive element so as to radiate the correct quantity of light onto the subject.
When there are several coexisting conditions, such as 1) the distance to the subject is small, 2) the reflectance of the subject is high, 3) the image-capturing sensitivity at the camera is high and 4) the photographic F value of the lens is set for bright light, the quantity of light irradiated on the subject immediately reaches the correct level after the light emission starts. In such a case, the drive element is switched to an OFF operation immediately after the drive element is turned on.
The maximum rated value of the collector current in a drive element such as an IGBT is normally determined in correspondence to the gate voltage. FIG. 3 illustrates the relationship between the gate voltage and the maximum rated collector current.
When the drive element is set to engage in an OFF operation immediately after it is turned on, as described above, the gate voltage of the drive element does not fully rise and instead, its level shifts in a relatively low voltage range slightly above the ON voltage. When the gate voltage is low, the maximum rated value for the collector current, too, stays at a low level as shown in FIG. 3.
The light emitting tube, on the other hand, is in an excited state immediately after the start of light emission in response to the ON operation of the drive element, and thus, the impedance within the tube is extremely low. In addition, a high voltage is applied to the two ends of the light emitting tube at the light emission start.
For these reasons, if the drive element shifts to an OFF operation immediately after light emission start, a large collector current is allowed to flow to the drive element despite a low maximum rated collector current level.
If the collector current of the drive element exceeds the maximum rated collector current even momentarily under those circumstances, problems arise, such as 1) the characteristics of the drive element deteriorate, 2) the service life of the drive element is reduced and 3) in the worst case scenario, the drive element breaks down, disabling light emission by the electronic flash device.
Accordingly, the applicant of the present invention disclosed in Japanese Laid-Open Utility Model publication No. H5-84947 an electronic flash device having a means for invalidating a light emission stop signal over a predetermined length of time after a light emission start signal is output and a means for invalidating the light emission start signal over a predetermined length of time after the light emission stop signal is output.
In the structure proposed in the related art, the light emission stop signal is invalidated over a predetermined length of time following an output of the light emission start signal. As a result, it is possible to avoid a state in which the maximum rated value is exceeded at the drive element as described above with a high degree of reliability, and a breakdown of the drive element and the like can be prevented.
Static electricity is generated in a dry environment during the winter, for instance. If a discharge pulse attributable to such static electricity should be applied to a circuit of an electronic flash device, the drive element may enter an electrically continuous state without an input of the light emission start signal, and start light emission.
Since the potential of such static electricity is extremely unstable, the switching operation at the drive element cannot be stabilized readily and thus, light emission may stop immediately after the start of the light emission. In such a situation, too, the current at the drive element may momentarily exceed the maximum rated value, which poses a concern for a breakdown of the drive element and the like.
In addition, there is another concern that the discharge pulse caused by the static electricity may be applied at a plurality of locations within. the circuits of the electronic flash device to allow a light emission start sequence and a light emission stop sequence to take place with a very small time lag in the circuit. In this case, too, the light emission may be stopped immediately after the light emission start, posing a concern for a breakdown of the drive element and the like.
In the device proposed in the related art (Japanese Laid-Open Utility Model Publication No. H 5-84947), the light emission stop signal is invalidated in response to an input of the light emission start signal. For this reason, it is difficult to fully address the problems caused by static electricity or the like (i.e., without an input of the light emission start signal) as described above.
An object of the present invention is to provide an electronic flash device capable of preventing a breakdown caused by an anomalous signal such as static electricity.
In order to attain the above object, an electronic flash device according to the present invention comprises: a flash light emission circuit that performs a flash light emission; a light emission control circuit that supplies power for light emission to the flash light emission circuit in response to an input of a light emission start signal and cuts off the power to the flash light emission circuit in response to an input of a light emission stop signal; and a protective circuit that detects a state that will induce power supply to the flash light emission circuit in response to an anomalous signal based upon a circuit operation of the light emission control circuit and prevents the power supply to the flash light emission circuit.
Another electronic flash device according to the present invention comprises: a flash light emission circuit that performs a flash light emission; a light emission control circuit that supplies power for light emission to the flash light emission circuit in response to an input of a light emission start signal and cuts off the power to the flash light emission circuit in response to an input of a light emission stop signal; and a protective circuit that detects a state that will induce power supply to the flash light emission circuit without an input of the light emission start signal based upon a circuit operation of the light emission control circuit and prevents the power supply to the flash light emission circuit.
In this electronic flash device, it is preferred that: the light emission control circuit includes at least a first switching element that turns on/off the power supply to the flash light emission circuit, a second switching element that outputs an ON signal for turning on the first switching element in response to an input of the light emission start signal and a third switching element that outputs an OFF signal for turning off the first switching element in response to the light emission stop signal; and the protective circuit prevents the first switching element from becoming turned on in response to the ON signal if the ON signal from the second switching element is detected while there is no input of the light emission start signal.
Another electronic flash device according to the present invention comprises: a flash light emission circuit that performs a flash light emission; a light emission control circuit that supplies power for light emission to the flash light emission circuit in response to an input of a light emission start signal and cuts off the power to the flash light emission circuit in response to an input of a light emission stop signal; and a protective circuit that detects a state that induces power supply to the flash light emission circuit in response to an anomalous signal based upon a circuit operation of the light emission control circuit and prevents a power supply cutoff to the flash light emission circuit.
Another electronic flash device according to the present invention comprises: a flash light emission circuit that performs a flash light emission; a light emission control circuit that supplies power for light emission to the flash light emission circuit in response to an input of a light emission start signal and cuts off the power to the flash light emission circuit in response to an input of a light emission stop signal; and a protective circuit that detects a state that induces power supply to the flash light emission circuit without an input of the light emission start signal based upon a circuit operation of the light emission control circuit and prevents cutoff of power supply to the flash light emission circuit.
In this electronic flash device, it is preferred that: the light emission control circuit includes at least a first switching element that turns on/off power supply to the flash light emission circuit and a second switching element that outputs an ON signal for turning on the first switching element in response to an input of the light emission start signal and outputs an OFF signal for turning off the first switching element in response to the light emission stop signal; and the protective circuit prevents the OFF signal from becoming output from the second switching element even if the light emission stop signal is input while the ON signal is output from the second switching element without an input of the light emission start signal. In this case, it is preferred that: the protective circuit includes a capacitor which is charged with the light emission start signal so as to detect an input of the light emission start signal.
Also, it is preferred that: the light emission control circuit includes at least a first switching element that turns on/off power supply to the flash light emission circuit and a second switching element that outputs an ON signal for turning on the first switching element in response to an input of the light emission start signal and outputs an OFF signal for turning off the first switching element in response to the light emission stop signal; and the protective circuit prevents the first switching element from becoming turned off if the ON signal from second switching element is detected while there is no input of the light emission start signal. In this case, it is preferred that: the light emission control circuit further includes a third switching element that turns off the first switching element regardless of whether the ON signal or the OFF signal is output from the second switching element when there has been an input of the light emission stop signal; and the protective circuit prevents the first switching element from becoming turned off by the third switching element regardless of whether or not there has been an input of the light emission stop signal if the ON signal is detected from the second switching element while there is no input of the light emission start signal. Furthermore, it is preferred that: the protective circuit includes a capacitor which is charged with the light emission start signal so as to detect an input of the light emission start signal.
In the above electronic flash devices, it is preferred that the protective circuit includes a speedup capacitor for hastening timing with which power supply prevention is started.